In recent years, the use of the ferroelectric materials for random access memory (RAM) elements has reached commercial applications in the semiconductor industry. Ferroelectric dielectrics typically have very high dielectric constants .about.500, which make these materials attractive as dielectrics for capacitors with high charge storage capacity. Ferroelectric memory elements are non-volatile, and programmable with low voltage, e.g. less than 5V. Other advantages include fast access times (&lt;40 ns), and robustness with respect to virtually unlimited numbers of read and write cycles. These memory elements also consume low power, are dense and exhibit radiation hardness.
Ferroelectric materials which have allowed this breakthrough in integrated circuit applications include perovskite structure ferroelectric dielectric compounds, for example, lead zirconate titanate PbZr.sub.x Ti.sub.1-x O.sub.3 (PZT), barium titanate (BT), and barium strontium titanate (BST).
Chapter E4.0 "Introduction to Ferroelectrics" co-authored by one of the present inventors, provides a general review of ferroelectrics, to be published in Handbook of thin film Technology (IOP Publishing, May 1995)
Much work has been devoted to developing low temperature processes for formation of thin films of ferroelectric dielectrics compatible with semiconductor processing for CMOS, bipolar and bipolar CMOS technologies. Integration of ferroelectric materials with monolithic integrated circuits brings challenges including the selection of compatible materials for bottom and top electrodes, and for barrier layers and capping layers, to avoid or control contamination problems. These issues are discussed in more detail, for example, in U.S. Pat. No. 5,330,931 "Method of making a capacitor for an integrated circuit" to Emesh et al. issued 19 Jul. 1994, and copending U.S. patent applications Ser. No. 08-348849 filed 28 Nov. 1994, to Leung and Emesh, entitled "Capacitor for Integrated circuit", and Ser. No. 08-348848 filed 28 Nov. 1994 to Emesh, McDonald and Chivukala, entitled "Method of forming crystalline ferroelectric dielectric material for an integrated circuit."
While conventional dielectrics such as silicon dioxide, silicon nitride and silicon oxynitride have functional characteristics which are substantially independent of frequency up to at least 10 GHz, the relatively low value of the dielectric constant (.epsilon..ltoreq.10) limits the capacitance attainable to about 2 to 3 fF/.mu.m.sup.2. On the other hand, the dielectric constant of ferroelectric dielectrics (typically .epsilon.&gt;500) allows for capacitances greater than 30 fF/.mu.m.sup.2. Consequently there is much interest in ferroelectric dielectric materials for small area capacitors, and for larger, high value capacitors. A number of applications require large on chip capacitances in the nF range. Ferroelectric dielectrics are of interest for applications as filter elements operating at low frequency (&lt;100 Hz) to GHz frequencies, and as coupling and decoupling capacitors. On the other hand, it is known that a relaxation mechanism causes the dielectric constant of many ferroelectric materials to decrease significantly to a low value at microwave frequencies.
Thus, for high frequency applications in the microwave (GHz) frequency range, barium strontium titanate (BST) is a preferred material. For example, Ueda et al. of Matsushita, in a publication IEEE Tokyo Section, Denshi Tokyo No. 32 (1993), presented results for GaAs based Microwave monolithic ICs (MMICs) comprising BST having a dielectric constant of about 300 formed by a sol-gel method, showing that BST has a flat dielectric response up to about 5 GHz, when fabricated at a temperature of 600.degree. C. The dielectric constant was increased to about 300 by annealing at higher temperature, .about.800.degree. C., but the frequency characteristic degraded significantly.
In a presentation by Gota Kano, of Matsushita Electronics Corp, and collaborators at the University of Colorado and Symetrix, at the International Symposium on Integrated ferroelectrics, Monterey, Mar. 9-11, 1992, the high frequency performance of thin films of several dielectric materials was compared. They reported that thin films of PZT, which in bulk ceramic form has a very high dielectric constant of about 1000, exhibits relaxation at much lower frequencies than BST, and beyond 100 MHz the dielectric constant of PZT drops to a very low value &lt;10. They concluded that this material was not useful as a capacitor material at high frequencies. Consequently manufacturers have focussed on other ferroelectrics, particularly BST, for high frequency applications. Nevertheless, at low frequencies, BST has a lower dielectric constant that PZT, and BST typically requires higher processing temperatures.